1. Field of the Invention
The present invention relates to a fixing apparatus of electromagnetic induction heating method and an image forming apparatus including the fixing apparatus.
2. Description of the Related Art
An image forming apparatus, such as a copying machine, includes a fixing apparatus for fixing toner onto a sheet by thermally fusing the toner transferred onto the sheet. In recent years, a fixing apparatus of an electromagnetic induction heating method has come into use, in which a rotary member (such as a fixing belt) made of thin-walled metal is induced to generate heat through induction heating using an exciting coil. According to the method, magnetic fluxes generated from the exciting coil are passed through a conductive portion of the rotary member so that eddy currents flow within the body of the rotary member, to thereby heat the rotary member through Joule heating generated by the eddy currents.
Further, the fixing apparatus of the electromagnetic induction heating method has magnetic cores disposed therein, in order to enhance convergence of the magnetic fluxes generated from the exciting coil. There has been proposed a fixing apparatus in which a positional relation between the magnetic cores and the exciting coil determines heating efficiency of the rotary member, and the heating efficiency is changed in part by displacing the magnetic cores (see Japanese Patent Application Laid-Open No. 2000-66543). According to the configuration, the magnetic core provided at an end portion of the rotary member is displaced, to thereby perform control so as to prevent heat generation in the end portion (temperature rise in the end portion) of the rotary member when small-sized sheets are continuously supplied.
However, according to the configuration disclosed in Japanese Patent Application Laid-Open No. 2000-66543, in a case where the magnetic core fails to be moved in the event of failure in a drive circuit for driving the magnetic core, the failure in the drive circuit cannot be detected. When the magnetic core disposed at the end portion of the rotary member fails to be moved in position while small-sized sheets are being continuously supplied, the temperature rise in the end portion of the rotary member occurs, with the result that the sheets get wrinkled due to the thermal expansion in the end portion of the rotary member. On the other hand, the productivity decreases if a down sequence of increasing intervals of conveying the sheets is performed in order to prevent the temperature rise in the end portion of the rotary member.
In view of the above, there is a method of providing a sensor for detecting whether or not the magnetic core has moved, on a drive shaft of the magnetic core. In such a case, however, the sensor needs to be disposed at a position close to the fixing apparatus, which means that an expensive heat-resistant sensor must be used.
Meanwhile, there is proposed an apparatus which includes a shutter configured to shield against the magnetic fluxes generated from the exciting coil (US 2006/0088328), in which the shutter is provided in a movable manner and shields against the magnetic fluxes at the end portion of the rotary member, to thereby prevent temperature rise in the end portion of the rotary member. However, even in this case, there is a problem that provision of a sensor for detecting whether or not the shutter has moved results in a cost increase.